如何在TreeView中选择Next/Previous TreeViewItem？

<TreeView x:Name="MyTreeView">
    <TreeViewItem Header="Item 1">
        <TreeViewItem Header="Item 1.1"/>
        <TreeViewItem Header="Item 1.2"/>
    </TreeViewItem>
    <TreeViewItem Header="Item 2">
        <TreeViewItem Header="Item 2.1"/>
        <TreeViewItem Header="Item 2.2"/>
    </TreeViewItem>
    <TreeViewItem Header="Item 3">
        <TreeViewItem Header="Item 3.1"/>
        <TreeViewItem Header="Item 3.2"/>
    </TreeViewItem>
</TreeView>

private void SelectNextTreeViewItem(TreeViewItem currentItem)
{
    if (currentItem == null) return;

    TreeViewItem nextItem = null;
    var parent = currentItem.Parent as TreeViewItem;

    if (parent != null)
    {
        var index = parent.Items.IndexOf(currentItem);
        if (index< parent.Items.Count - 1)
        {
            nextItem = parent.Items[index + 1] as TreeViewItem;
        }
    }
    else
    {
        var index = MyTreeView.Items.IndexOf(currentItem);
        if (index < MyTreeView.Items.Count - 1)
        {
            nextItem = MyTreeView.Items[index + 1] as TreeViewItem;
        }
    }

    if (nextItem != null)
    {
        MyTreeView.SelectedItem = nextItem;
        nextItem.IsSelected = true;
        nextItem.Focus();
    }
}

private void SelectPreviousTreeViewItem(TreeViewItem currentItem)
{
    if (currentItem == null) return;

    TreeViewItem previousItem = null;
    var parent = currentItem.Parent as TreeViewItem;

    if (parent != null)
    {
        var index = parent.Items.IndexOf(currentItem);
        if (index > 0)
        {
            previousItem = parent.Items[index - 1] as TreeViewItem;
        }
    }
    else
    {
        var index = MyTreeView.Items.IndexOf(currentItem);
        if (index > 0)
        {
            previousItem = MyTreeView.Items[index - 1] as TreeViewItem;
        }
    }

    if (previousItem != null)
    {
        MyTreeView.SelectedItem = previousItem;
        previousItem.IsSelected = true;
        previousItem.Focus();
    }
}

private void NextButton_Click(object sender, RoutedEventArgs e)
{
    var selectedItem = MyTreeView.SelectedItem as TreeViewItem;
    SelectNextTreeViewItem(selectedItem);
}

private void PreviousButton_Click(object sender, RoutedEventArgs e)
{
    var selectedItem = MyTreeView.SelectedItem as TreeViewItem;
    SelectPreviousTreeViewItem(selectedItem);
}